From U.S. Pat. No. 7,012,409 (Schreiber) a power converter circuit for a generator with dynamically variable power output is previously known. The power converter circuit comprises a wind power system connected to a grid point of an electric network. The object of the power converter circuit is to provide an expanded connection of a generator that outputs dynamically variable power to the electric network. A further object is to introduce a triggering method by which reactive power may be fed into the electric network in the event of a short circuit.
The known electric power circuit comprises two individual connections to the network. A first connection comprises a direct connection to the network by which electric power is fed during high wind speeds when the wind mill is capable of generating power at a constant speed. A second connection comprises a first and second converter connected in a back-to-back configuration with a capacitor. By this connection also power generated at low wind speeds, when the wind mill is controlled at a varying speed, may be fed into the network. The first and second converter comprises voltage source converters (VSC) containing self commutating power electronics (PE).
The triggering method comprises three operation modes following a short circuit in the network. In the event of a network short circuit the generator is automatically disconnected from the electrical network and during a first period of time the second converter outputs reactive power from the capacitor to the network. During a second period of time the second converter draws operating power from the network and feeds reactive power back to the network. Finally, during a third period of time when the generator is still disconnected the converter system controls the gentle restart of the generator until the generator again furnish energy to the electrical network.
In the known electric power circuit the wind mill is automatically disconnected in an event of short circuit in the network. This disconnection implies on the one hand that the rotating system, containing a considerable moment of inertia, must be decelerated by peripheral forces. On the other hand the wind mill looses the synchronization to the network which implies a time consuming restart of the generator.
From WO 2005/062438 an electric power network is previously known, the object of which is to control the power transmission in a power network during a fault condition. This object is achieved by introducing at a fault condition a voltage raising means between a rotating electric machine and its connection to the network. On sensing a fault condition in the network the voltage raising means are introduced to prevent the normal protecting devices to respond and disconnect the rotating electric machine from the network.
In a normal generating mode of the wind mill the produced energy is in balance with the power consumption of the network. Hence the rotating mechanical force from the wind mill is balanced by the electromotive force (emf) from the network. A sudden disconnection of the generator from the network interrupts this force balance and the wind rotor starts free wheeling. The rotating part of a wind mill containing a rotor with vanes, a gear box and the rotor of the generator exhibits when rotating a considerable moment of inertia. When the wind mill is suddenly disconnected from the network the moment of inertial will cause the speed of the wind mill rotor to increase. Since also the wind mill continues to introduce wind power to the rotor the rotation speed will further increase.
This increase in rotation may be retarded by mechanical brakes and by regulating the pitch of the vanes. However these mechanical regulating systems are slow. Due to time consuming regulation of the wings of the wind mill and due to the fact that the wind is still transferring wind energy to the wind mill the speed of the rotor is further increased. By the time the rotor experience retardation from the mechanical regulation the rotor speed may already have reached hazardous levels.
There are known a plurality of ways to reduce the speed of the disconnected wind mill rotor. Most of these methods convert mechanical energy into heat which is dissipated to the air. However there is no control of the wind mill once it has been disconnected from the network. Thus the synchronization is lost and therefore the wind mill has to be started from scratch. This is time consuming and during this time the wind mill will not generate power to the network.
There is thus a need within the wind power industry to maintain the power generation as long as possible in a short circuit situation on the network.